Polymer beads are usually produced by free radical suspension polymerization, a well known process developed in 1909. See Hofman et al., Ger. Pat. 250,690 (1909). Porous polymer beads based on, for example, poly(styrene-co-divinylbenzene) are widely used in the synthesis of ion-exchange resins. In addition, significant speciality applications exist for the use of such polymer beads in areas related to separation science. These applications include separation of enantiomers from racemic mixtures, analysis of blood samples, water purification, high-performance liquid chromatography, and the like.
Some of these speciality applications require or would benefit from the use of polymer beads which have unique surface properties. For example, when a conventional column containing hydrophobic polymer beads is used in the analysis of human plasma, the proteins in the blood plasma are denatured on the hydrophobic surface of the beads. This eventually destroys the column through clogging which increases the back pressure to such an extent that the column must often be replaced after only a very short useful lifetime. In contrast, a column packed with polymer beads having a hydrophobic core and a hydrophilic outer layer does not suffer from the same problem because the hydrophilic surface does not react with blood protein so as to cause clogging of the column.
Another speciality application is the separation of enantiomers from racemic mixtures. Such chromatographic separations require the use of beads with a chiral surface layer. These separations are important for the analysis, identification, or preparation of optically active compounds.
Other possible applications for which polymer beads having a hydrophilic or chiral outer surface include catalysis in which it is sometimes desirable to segregate an active component at the surface of a solid catalyst. The selection of a reactive chiral or hydrophilic group and its placement at the surface of a polymer bead could accomplish such an active component segregation. In toner chemistry, it is often desirable to modify the surface chemistry of toner particles which are generally non-porous non-cross-linked polymer particles. Currently this is performed through the use of simple mixtures or coatings of the toner particles with the desired additives.
While some polymer beads with hydrophilic surfaces are available, the processes used to produce such polymer beads are difficult to perform, are limited in the types of monomers and groups which they can introduce, and are in many cases not economical. As regards chiral surface groups, polymer beads having chiral surface functionality and hydrophobic cores and processes for producing such are not known in the prior art. Accordingly, the speciality applications discussed previously, which would benefit from the use of such polymer beads, remain as theoretical applications for polymer beads without suitable polymer beads to perform the operation needed by such.
The production of polymer substrates, particles and/or beads having hydrophilic cores and hydrophilic outer layers or hydrophilic surface functionality is disclosed in a number of publications. For example, U.S. Pat. No. 4,571,390 discloses a porous styrene polymer substrate wherein the surface is rendered hydrophilic by chloromethylating it to introduce methylol groups onto the polymerized substrate. The substrate is disclosed as useful in adsorbing high molecular weight proteins. U.S. Pat. No. 4,898,913 is directed to a process for altering a macroporous crosslinked hydrophobic copolymeric lattice produced by a precipitation polymerization of ester monomers. According to the method described therein, the copolymeric lattice, after formation, must be separated from the mixture in which it is formed, and then, in a separate operation, the surface of the copolymer is rendered hydrophilic either by reaction with an aqueous alkali or by a second polymerization using a hydrophilic acrylate monomer.
European Patent No. 0371258 discloses porous polymer substrates comprising an acrylonitrile polymer or copolymer core with a hydrolized surface layer having post-generated amide surface groups. The porous substrates such as beads are disclosed as being useful in chromatography separation processes. The amide surface groups are produced by adding a peroxide to a polymer suspension and heating for a time sufficient to convert about 15 mole percent of the total surface nitrile groups to amide groups. This process and the resulting product are limited to specific types of surface functional groups.
Another polymer bead material having a hydrophilic surface is disclosed in U.S. Pat. No. 4,882,226. The polymer beads comprise (i) a core material obtained by the addition polymerization of monomers comprising methacrylic acid and (ii) a hydrophilic coating which is covalently bonded to the core as a result of complete or partial conversion of the carboxyl function with a compound containing at least three carbon atoms and an epoxy group. The formation of the covalent bond to create the hydrophilic surface is a multi-step and complex process.
An expensive and complicated process for producing polymer microspheres is taught in U.S. Pat. No. 4,170,685. The process involves the production of microspheres by the use of ionizing radiation. Hydrophilic characteristics are provided by addition of a suitable unsaturated comonomer. See also U.S Pat. No. 4,259,223 which discloses a similar process.
Japanese Patent No. 62046260 discloses still another polymer bead material having a hydrophobic core and a hydrophilic surface layer to which a polyethylene fiber is laminated. The multilayer material is disclosed as being useful in determining components in a blood serum solution.
The prior art processes for forming polymer particles with hydrophobic cores and hydrophilic surface layers are either complicated and uneconomical or limited to a specific surface functionality. Moreover, none of the prior art processes describes a method for introducing a chiral functional group to the surface of hydrophobic polymer beads.
Accordingly, it is one of the objects of the present invention to develop an improved process for introducing hydrophilic or chiral surface functionality on hydrophobic porous polymeric particles.
It is another object of the present invention to produce polymeric particles that have chiral groups on their surface.
It is a further object to manufacture polymeric particles from solid monomers or monomers having a low solubility in the polymerization solvent.